Systems and methods for catalyst screens in selective catalytic reduction reactors

ABSTRACT

A system for use in selective catalytic reduction reactor is disclosed. The system may include a catalyst bed and a screen located close to the catalyst bed in a manner so that flow of flue gas to the catalyst bed contacts the screen before it contacts the catalyst bed. The screen may be adapted to support a weight of at least 400 pounds above the catalyst bed so that the weight is not imposed on the catalyst. The screen may have a plurality of holes across its surface in a manner so that the screen is adapted to change the velocity distribution of the flue gas as it flows through the screen.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 15/388,401, filed Dec. 22, 2016 and entitled “SYSTEMS ANDMETHODS FOR CATALYST SCREENS IN SELECTIVE CATALYTIC REDUCTION REACTORS,”the disclosure of which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The present invention is related to selective catalytic reductionreactors. More specifically, the present invention is related to systemsand methods for providing screens to protect catalyst in selectivecatalytic reduction reactors.

BACKGROUND OF THE INVENTION

Selective catalytic reduction reactors are used to convert nitrousoxides (NO_(x)), to nitrogen (N₂) and water (H₂O). The selectivecatalytic reduction process is typically carried out to prevent thenitrous oxides from entering and polluting the atmosphere. Nitrousoxides are produced by different types of industrial equipment such asboilers, engines, and turbines. For example, in power stations that usecoal to generate electricity, flue gases emanating from boilers in thepower stations contain nitrous oxides.

The selective catalytic reduction process involves adding a reductant tothe flue gas and passing the flue gas through a bed of catalyst toconvert the nitrous oxides. The reductant may include ammonia or ureaand the catalysts may include zeolites, metal oxides such as vanadiumoxide and titanium oxides, and the like.

For boilers powered by coal, ash produced as a result of burning thecoal may be transported by the flue gas to the catalyst, where the ashmay bind to and plug the catalyst. Ash typically comprises silicondioxide, calcium oxide, carbon and many other constituents depending onthe makeup of the coal being burned. The combustion ash particles areusually small (up to 300 micro meters in diameter) and so they areeasily suspended in the flue gas. However, the combustion ash particlescan form large particle ash (LPA), which may have a diameter exceeding 1centimeter. The LPA can plug the openings in the catalyst. Thus, screensare provided to remove LPA particles from the flue gas before they getto the catalyst. For example, patent application Ser. No. 13/633,717,entitled, “Apparatus and Methods for Large Particle Ash Separation FromFlue Gas Using Screens Having Semi-Elliptical Cylinder Surfaces,” filedOct. 2, 2012 to Buzanowski et al., describes screens for separating ashparticles from the flue gas. In addition to such screens for separatingLPA from the flue gas, selective catalytic reduction reactors areprovided with protective catalyst wire mesh screens that are placedabove the catalyst. The catalyst is a very expensive component of theselective catalytic reactor and the catalyst wire mesh screen isdesigned to allow personnel, when the reactor is not in operation, towalk in the reactor, on top of the catalyst wire mesh screen, withoutdamaging the catalyst. In some instances, the catalyst wire mesh screenis designed to have slightly smaller openings than the openings in thecatalyst so that there is a buildup of ash particles from the flue gason the catalyst wire mesh screen instead of the catalyst.

FIG. 1 shows prior art selective catalytic reduction reactor system 10.Selective catalytic reduction reactor system 10 shows equipment relatedto a selective catalytic reduction process in a typical coal fired powerplant. In boiler 100, coal is mixed with air (from preheater 101) andburned. The burning coal causes an increase in temperature in boiler 100so that water injected into boiler 100 is vaporized to form steam. Theburning coal produces ash particles 102 (including LPA 102A and fly ash102B), which flows with hot flue gas 103 through duct 104A. Duct 104Aleads to LPA screen separator 105. LPA screen separator 105 has holeshaving a diameter so that flue gas 103 and fly ash 102B pass through LPAscreen separator 105. However, LPA 102A particles are too big to passthrough the holes of LPA screen separator 105.

Because these LPA 102A particles are too big to pass through the holesof LPA screen separator 105, they accumulate in hopper 106. Flue gas 103and fly ash 102B passes through LPA screen separator 105 and enters duct104B. Duct 104B channels flue gas 103, fly ash 102B, and reductant 107to selective catalytic reduction (SCR) reactor 108. SCR reactor 108removes nitrous oxides from flue gas 103 by converting the nitrousoxides to nitrogen and water in a reduction reaction. Catalyst bed 109facilitates this conversion by speeding up the reduction reaction whenflue gas 103, fly ash 102B, and reductant 107 are passed throughcatalyst bed 109. Flue gas 112, leaving catalyst bed 109, has a reducedamount of NO_(x) compared with flue gas 103 and is discharged into theatmosphere or cleaned further and then discharged into the atmosphere.

Ash particles small enough to pass through LPA screen separator 105(e.g., fly ash 102B) may accumulate on catalyst wire mesh screen 110 orin SCR catalyst bed 109. The accumulation of fly ash 102B on catalystwire mesh screen 110 and/or in SCR catalyst bed 109 may negativelyaffect the performance of SCR catalyst bed 109 and the catalyticreduction process. Thus, when fly ash 102B accumulates sufficiently oncatalyst wire mesh screen 110, cleaning equipment 111 may be used toclear away those ash particles. Cleaning equipment 111 may include aircannon cleaning equipment and/or sonic horn cleaning equipment.

The sonic horn is a low frequency, high energy acoustic horn used as acleaning mechanism. When the sonic horn emits low frequency sound, thesound waves vibrate fly ash (e.g., fly ash 102B) and dislodge it fromwhere it has settled. If the sound doesn't dislodge fly ash 102B, itjust vibrates and gets packed into the catalyst wire mesh screen.Because of the rough finish of catalyst wire mesh screen 110, the sonichorns tend to displace and lodge fly ash 102B into catalyst wire meshscreen 110 and create a buildup of the fly ash. If the sonic hornssufficiently vibrate and dislodge fly ash 102B particles, then eithergas flow or gravity moves the fly ash 102B deposits away from catalystwire mesh screen 110. But when the sonic horns are not able to vibratefly ash 102B particles enough to get them off catalyst wire mesh screen110, then fly ash 102B particles get packed in and on catalyst wire meshscreen 110.

Generally, fly ash 102B particles are smaller than the holes in catalystwire mesh screen 110. Catalyst wire mesh screen 110 is designed so thatmost of fly ash 102B particles flow through the mesh wire and throughthe catalyst. However, because of the rough surfaces on catalyst wiremesh screen 110, fly ash 102B particles get attached to the roughsurfaces and then fly ash 102B particles are packed on catalyst wiremesh screen 110 by the sonic horns.

The performance of SCR catalyst bed 109 may also be affected by unequalflue gas velocity flowing through SCR catalyst bed 109. Such unequalflue gas velocity flow may erode SCR catalyst bed 109. High velocityflue gas flow through the catalyst erodes and damages the catalyst. Onthe other hand, if the flue gas velocity is too low, it may cause flyash 102B particles to accumulate on catalyst wire mesh screen 110. Thismay cause a buildup of piles of fly ash particles above catalyst bed 109on catalyst wire mesh screen 110. Such fly ash piles can cause angledflow of the flue gas and can increase velocity of the flue gas, both ofwhich may cause or exacerbate erosion of the catalyst in SCR catalystbed 109. Further, uneven velocity distribution may affect residency andeffectiveness of catalyst to fully catalyze chemical reaction.

BRIEF SUMMARY OF THE INVENTION

A discovery has been made of a screen for use in selective catalyticconverter reactors to protect the catalyst in the selective catalyticconverter reactors whilst adjusting the flow of flue gas to the catalystas it passes through the screen. Adjusting the flow of the flue gasflowing to the catalyst may involve adjusting the velocity distributionof the flue gas as it flows through the screen towards the catalyst. Thescreen may also be adapted to have a smooth surface, which resists flyash accumulation and improves the effectiveness of air cannon and sonichorn cleaning processes implemented in selective catalytic converterreactors. Further, the screen may have a domed shape, which resists flyash accumulation on the screen.

In existing systems, potential failure of a primary LPA Screen (e.g.,LPA screen separator 105) may cause LPA to pass through to the reactor.Screens according to embodiments of the invention, can act as anadditional protection barrier for the catalyst. Further, LPA are usuallyuneven in geometry. Some particles may be round and some may be long andthin. Particles may pass through the primary LPA screen because of this.Screens, according to embodiments of the invention, can act as anadditional protection barrier for the catalyst.

Embodiments of the invention include a system for use in a selectivecatalytic reduction reactor. The system may include a catalyst bed and ascreen located at a distance in a range of 1 in. to 12 ft. above thecatalyst bed. In this way, flue gas flowing to the catalyst bed contactsthe screen or passes through the screen before it contacts the catalystbed or passes through the catalyst bed. The screen may be adapted tosupport a weight of at least 400 pounds above the catalyst bed so thatthe weight of the body is not imposed on the catalyst of the catalystbed. The screen has a plurality of holes across its surface. Overallvelocity distribution of the flue gas flowing through the screen can bequantified in terms of Root Mean Square (RMS). The screen may be adaptedto change the velocity distribution of the flue gas as the flue gasflows through the screen, where the change in velocity distribution bythe screen results in a 3 to 6% decrease or improvement in RMS of theoverall velocity distribution of the flue gas.

Embodiments of the invention include a method for protecting a catalystbed in a selective catalytic reduction reactor. The method may includedisposing a screen at a distance in a range of 2 in. to 12 ft. above thecatalyst bed so flue gas flowing to the catalyst bed contacts the screenor passes through the screen before it contacts the catalyst bed orpasses through the catalyst bed. Overall velocity distribution can bequantified in terms of RMS. The screen may be adapted to withstand aweight of at least 400 pounds without that weight being imposed on thecatalyst of the catalyst bed. The screen has a plurality of holes acrossits surface. The method may also include flowing the flue gas throughthe screen. Further, the method may include changing the velocitydistribution of the flue gas as it flows through the screen, wherein thechange in velocity distribution by the screen results in a 3 to 6%decrease or improvement in RMS of the overall velocity distribution ofthe flue gas.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription that follows may be better understood. Additional featuresand advantages will be described hereinafter which form the subject ofthe claims. It should be appreciated by those skilled in the art thatthe conception and specific embodiment disclosed may be readily utilizedas a basis for modifying or designing other structures for carrying outthe same purposes of the present application. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the application as set forth in theappended claims. The novel features which are believed to becharacteristic of embodiments described herein, both as to itsorganization and method of operation, together with further objects andadvantages will be better understood from the following description whenconsidered in connection with the accompanying figures. It is to beexpressly understood, however, that each of the figures is provided forthe purpose of illustration and description only and is not intended asa definition of the limits of the present embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding, reference is now made to thefollowing descriptions taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 shows a prior art selective catalytic reduction reactor system;

FIG. 2 shows a selective catalytic reduction reactor system, accordingto embodiments of the invention;

FIGS. 3A and 3B show screens for use in a selective catalytic reductionreactor system, according to embodiments of the invention;

FIG. 4 shows a velocity distribution graph for a catalyst wire meshscreen;

FIG. 5 shows velocity distribution graph for a flat metallic catalystplate screen, according to embodiments of the invention; and

FIG. 6 shows a method for protecting a catalyst bed in a selectivecatalytic reduction reactor, according to embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A discovery has been made of a catalyst screen for use in selectivecatalytic converter reactors that protect catalyst in the selectivecatalytic reduction reactors whilst solving the aforementioned problemswith convention catalyst wire mesh screens. In embodiments of theinvention, the flow of the flue gas to the catalyst is adjusted by thecatalyst screen. Adjusting the flow of the flue gas to the catalyst mayinvolve adjusting the velocity distribution of the flue gas as it flowsthrough the screen towards the catalyst. Adjusting the velocitydistribution in this way can help the catalyst bed to operate moreefficiently. The catalyst screen may also be adapted to have a smoothsurface, which causes the screen to resist fly ash accumulation andimproves the effectiveness of air cannon and sonic horn cleaningprocesses implemented in selective catalytic converter reactors.Further, the catalyst screen may also have a domed shape, which alsohelps to repel the buildup of fly ash on the catalyst screen.

FIG. 2 shows selective catalytic reduction reactor system 20, accordingto embodiments of the invention. Selective catalytic reduction reactorsystem 20 shows equipment related to a selective catalytic reductionprocess of a coal fired power plant, according to embodiments of theinvention. In boiler 200, coal is mixed with air (from preheater 201)and burned. The burning coal causes an increase in temperature in boiler200 so that water injected into boiler 200 is vaporized to form steam.The burning coal produces ash particles 202 (including LPA 202A and flyash 202B), which flows with hot flue gas 203 through duct 204A. Duct204A leads to LPA screen separator 205. LPA screen separator 205 hasholes having a diameter so that flue gas 203 and fly ash 202B passthrough LPA screen separator 205. However, LPA 202A particles are toobig to pass through the holes of LPA screen separator 205.

Because these LPA 202A particles are too big to pass through the holesof LPA screen separator 205, they accumulate in hopper 206. Flue gas203, along with fly ash 202B, passes through LPA screen separator 205and enters duct 204B. Reductant 207 may be injected into duct 204B. Duct204B channels flue gas 203, fly ash 202B, and reductant 207 to SCRreactor 208. SCR reactor 208 removes nitrous oxides from flue gas 203 byconverting the nitrous oxides to nitrogen and water in a reductionreaction. Catalyst bed 209 facilitates this conversion by speeding upthe reduction reaction when flue gas 203, fly ash 202B, and reductant207 are passed through catalyst bed 209. Flue gas 212, leaving SCRcatalyst bed 209, has a reduced amount of NO_(x) compared with flue gas203 and is discharged into the atmosphere or cleaned further and thendischarged into the atmosphere. Fly ash 202B flows with the flue gasthrough the catalyst and is collected by either a bag house or aprecipitator.

In embodiments of the invention, ash particles small enough to passthrough LPA screen separator 205 (e.g., fly ash 202B) may contactcatalyst plate screen 210. However, catalyst plate screen 210 is adaptedso that it reduces the tendency for fly ash 202B to accumulate thereonas compared to when fly ash 102B contacts catalyst wire mesh screen 110.In embodiments of the invention, catalyst plate screen 210 has a smoothsurface so that the friction between fly ash 202B and the surface ofcatalyst plate screen 210 is sufficiently low so that fly ash 202Beasily slides off and does not accumulate on catalyst plate screen 210.And if some fly ash 202B does accumulate on catalyst plate screen 210,the low friction surface of catalyst plate screen 210 is such that it ismuch easier for cleaning equipment 211 to clear away fly ash 202B ascompared to when fly ash 102B is deposited on catalyst wire mesh screen110. Cleaning equipment 211 may include canon cleaning equipment andsonic horn cleaning equipment.

In embodiments of the invention, the performance of SCR catalyst bed 209may be improved by normalizing unequal gas velocity flowing through SCRcatalyst bed 209. In this way, the possibility of erosion of SCRcatalyst bed 209 as a result of unequal gas velocity flow through SCRcatalyst bed 209 is eliminated or at least reduced.

FIGS. 3A and 3B show catalyst plate screens 30 for use in a selectivecatalytic reduction reactor according to embodiments of the invention.Catalyst plate screens 30 are adapted to support a weight of at least400 pounds. Catalyst plate screens 30 include a plurality of holesacross its surface. Catalyst plate screens 30 are adapted to change thevelocity distribution of flue gas as the flue gas flows through catalystplate screens 30. Catalyst plate screens 30 may be configured so thatthe change in velocity distribution as a result of catalyst platescreens 30 comprises a 3 to 6% decrease or improvement in RMS of theoverall velocity distribution of the flue gas.

Catalyst plate screens 30 may be made of materials such as metal (e.g.different types of steel), composite materials (e.g., carbon fibercomposites, engineered materials, etc.), polymers and/or the like, andcombinations thereof. In embodiments of the invention, catalyst platescreens 30 may include a perforated metal plate. For example, inembodiments of the invention, catalyst plate screens 30 may be made from12 gauge carbon steel. In embodiments of the invention, catalyst platescreens 30 may include surface 300A and/or 300B, which includes holes302 within top portion 303. A difference between catalyst plate screen30 shown in FIG. 3A and catalyst plate screen 30 in FIG. 3B is thatsurface 300A is a flat surface while surface 300B is a dome shapedsurface. Top portion 303 is supported by side walls 301. Top portion 303may be connected to side walls 301 by methods such as welding, bolting,and the like. Catalyst plate screens 30 are shown in a square shape inFIGS. 3A and 3B (considering a top view); however, catalyst platescreens 30 may have other shapes such as rectangular, circular etc. Inembodiments of the invention, catalyst plate screens 30 may each cover asurface area of 10 sq. ft. to 84 sq. ft. e.g., having a length in therange 2 ft. to 6 ft. and width in the range 5 ft. to 14 ft., platethickness in the range of 10 gauge to 16 gauge, and side wall height of1 in. to 12 in.

In embodiments of the invention, side walls 301 may be adapted to reston an area surrounding a catalyst bed (e.g., SCR catalyst bed 209 in aSCR reactor 208) so that surface 300A or 300B is above the catalyst bed.In this way, catalyst plate screens 30 can support one or more loadsabove the catalyst bed without the one or more loads being imposed onthe catalyst bed. For example, in embodiments of the invention, catalystplate screens 30 are able to support the weight of a person and/orequipment so that the person can walk on and/or equipment can be movedon catalyst plate screens 30 without the weight of the person or theequipment impacting the catalyst bed. Hence, catalyst plate screens 30provide a protective function to the catalyst bed.

In embodiments of the invention, catalyst plate screen 30 changes gasflow distribution as a result of the perforated plate structureproviding a sufficient flow impacting surface and appropriate holesizes. The flow impacting surface area of a screen is the surface areaof the screen that a gas impacts when the gas is flowing perpendicularto the screen. For example, for a flat perforated plate screen, flowimpacting surface area is the surface area of one side of the perforatedplate screen (e.g., surface area of surface 300A or 300B of catalystplate screens 30) minus the surface area of the holes in the screen.

In embodiments of the invention, catalyst plate screen 30 is aperforated plate structure and the flue gas flows and impacts, in aperpendicular or substantially perpendicular direction, surface 300A orsurface 300B, which causes the flow vectors to separate and changedirection as the flue gas accelerates through holes 302. Therefore,planar velocities are found on the surface of the screen which resultsin flue gas velocity normalization and helps to distribute the overallflow more evenly.

Further, in embodiments of the invention, the hole sizes of catalystplate screen 30 can be adjusted to cause higher backpressure on somelocations and lower backpressure on other locations. The resultingvelocities through catalyst plate screen 30 are directly proportional tothe backpressure created. Therefore, the velocity distribution throughthe screen can be altered.

Overall, in embodiments of the invention, the “plate with holes”structure of catalyst plate screen 30 acts as a pressure barrier whichbreaks-up and normalizes flow while still maintaining a low overallpressure differential, as opposed to typical catalyst wire mesh screens,which simply lets flue gas flow through at the velocity andconcentration of the flue gas as it approaches the catalyst wire meshscreens.

In embodiments of the invention, holes 302 in surface 300A and surface300B may have shapes such as the shape of a hexagon, circle, square,rectangular, triangle, pentagon, the like, and combinations thereof. Inembodiments of the invention, holes 302 may be formed by perforating topportion 303 (including surface 300A or surface 300B). Alternatively oradditionally, in embodiments of the invention, holes 302 may be formedwhen top portion 303 is created e.g., from a molding process in whichthe mold defines the formation of holes 302. In embodiments of theinvention, top portion 303 may include different hole sizes arranged ina manner to achieve a change in the velocity distribution of a gas, suchas a flue gas, flowed through catalyst plate screen 30. In embodimentsof the invention, holes 302 include holes of different sizes, e.g., 2.5mm. to 7.5 mm. arranged to achieve the change in the velocitydistribution of the flue gas. In embodiments of the invention, multiplescreens of different hole sizes may be combined to strategically changevelocity distribution. For example, if a certain reactor experienceshigh velocities at its outer edges and requires 100 screens to cover theentire area, then 50 screens of hole sizes 2.5 mm to 4.5 mm. diameter(e.g., 3.5 mm. diameter) may be used on the outer edges and 50 screensof hole sizes 4.6 mm to 7.5 mm. diameter (e.g., 5.5 mm. diameter) in thecenter. In other words, if the velocity of gas flow is high on the outeredges of the catalyst, the screen may be adapted to have smaller holeson the outer edges (near the perimeter of the screen) in comparison toholes at the inner section of the screen (away from the perimeter). Theconverse may also be implemented so that if a certain reactorexperiences low velocities at its outer edges and requires 100 screensto cover the entire area, then 50 screens of 4.6 mm to 7.5 mm. diameter(e.g., 5.5 mm. diameter) may be used on the outer edges and 50 screensof hole sizes 2.5 mm to 4.5 mm. diameter (e.g., 3.5 mm. diameter) in thecenter. In other words, if the velocity of gas flow is low on the outeredges of the catalyst, the screen may be adapted to have larger holes onthe outer edges (near the perimeter of the screen) in comparison toholes at the inner section of the screen (away from the perimeter). Inembodiments of the invention, the ratio of area of flow impactingsurface area/holes of the screen is in the range of 45% to 65%.

In embodiments of the invention, catalyst plate screen 30 is adapted tonormalize gas flow that has different velocities so that catalyst platescreen 30 increases the velocity of a section of the gas flowing tocatalyst plate screen 30 at a velocity lower than the gas's averagevelocity and decreases the velocity of a section of the gas flowing tocatalyst plate screen 30 with a velocity higher than the gas's averagevelocity. The representations of velocity distribution shown in FIG. 4and FIG. 5 are from computation flow dynamic analysis comparing flow ofgas through a catalyst wire mesh screen and a perforated metal platescreen. FIG. 4 shows velocity distribution graph for a catalyst wiremesh screen. FIG. 5 shows velocity distribution graph for a flatmetallic plate screen, according to embodiments of the invention.

Computational Flow Dynamic (CFD) analysis was employed to study theeffects of different screens. A baseline study was performed on astandard woven #3 Mesh which is typically installed above the topcatalyst layer in typical SCR systems. The study was performed with anincoming velocity gradient which ranged from 5 ft./s on the left to 25ft./s on the right along the x-axis. The initial CFD simulation showsthe velocity profile is nearly unchanged as it impinges the #3 Meshshowing almost no normalization of the flue gas on the SCR catalyst.However, when catalyst plate screen 30 is used, it normalizes the flow.In other words, it increases low velocity areas in the flow anddecreases high velocity areas in the flow. This is advantageous to theoperation of the SCR. For example, existence of low flow areas isdisadvantageous because it results in accumulation of fly ash particles.On the other hand, if the velocity of the flue gas is too high, itcauses erosion of the catalyst. Also, uneven velocity distributionthrough the catalyst reduces the effectiveness of the catalyst to reactequally with the flue gas.

Thus, in embodiments of the invention, catalyst plate screen 30increases velocity of the flue gas in the low flow areas and decreasesthe high velocity of the high flow areas so that the velocity of allareas of the flow is closer to the average velocity of the flow. Inother words, catalyst plate screen 30 redistributes the gas flow, whichtranslates to an improvement in the reduction of fly ash buildup in theSCR.

In embodiments of the invention, top portion 303 is adapted to have asmooth surface (e.g., surface 300A and surface 300B) characterized byperforated plate with a roughness of less than 20 μm in R_(a) (an R_(a)of 20 μm). Because surface 300A and surface 300B are smooth, catalystplate screen 30 resists the buildup of ash particles and so it is easierto clear away any ash particles deposited on catalyst plate screen 30 bycleaning methods such as methods that use air cannons and sonic horns.In embodiments of the invention in which catalyst plate screen 30 is adomed screen, as shown in FIG. 3B, surface 300B is a smooth surface andhas a dome shape that cooperate to resist buildup of ash particles oncatalyst plate screen 30. Catalyst plate screen 30 having domed screensurface 300B makes it easier for fly ash particles to slide off catalystplate screen 30. In other words, configuring catalyst plate screen 30 tohave domed screen surface 300B with a low friction surface may have acombined effect of causing the fly ash particles to slide easily fromcatalyst plate screen 30.

Embodiments of the invention improve the performance of cleaningequipment such as the sonic horns, and air cannons, by adapting surfaces300A and 300B to be smooth. In this way, when the cleaning mechanismsvibrate the ash particles, this vibration of ash particles on the smoothsurface moves the ash particles easier versus the rough finish of thecontemporary catalyst wire mesh screen (e.g., catalyst wire mesh screen110). Thus, embodiments of the invention are adapted to improve theperformance of the sonic horns and to improve the effectiveness of theair cannon blast. By reducing the friction across catalyst plate screen30 it is possible to keep the air cannon blast on the screen longer andprovide additional cleaning.

FIG. 6 shows method 60 for protecting a catalyst bed in a selectivecatalytic reduction reactor, according to embodiments of the invention.Method 60 may involve using catalyst plate screens described herein,such as catalyst plate screen 30. Method 60 may include, at block 600,disposing a screen at a distance in a range of 1 in. to 12 ft. above thecatalyst bed so that flow of flue gas to the catalyst bed contacts thescreen or passes through the screen before it contacts the catalyst bedor passes through the catalyst bed. The screen may be adapted towithstand a weight of at least 400 pounds without that weight beingimposed on the catalyst. The screen has a plurality of holes across itssurface. Method 60 may then include, at block 601, flowing the flue gasthrough the screen. At block 602, method 60 may further include,changing the velocity distribution of the flue gas as it flows throughthe screen, wherein the change in velocity distribution as a result ofthe screen comprises 3 to 6% decrease or improvement in RMS of theoverall velocity distribution.

Overall, there are several benefits to implementing catalyst platescreens to protect catalyst beds in selective catalytic reductionreactors according to embodiments of the invention as compared withconventional catalyst wire mesh screens. For example, use of catalystwire mesh screens result in the buildup of piles of fly ash on thecatalyst wire mesh screens over time. In contrast, the catalyst platescreen, according to embodiments of the invention, resists ash build upas a result of the smoother surface, that interfaces with the fly ashparticles and/or, in the case of some embodiments, the dome shape of thecatalyst plate screen. The smoother surface and the dome causes the flyash particles to slide off the catalyst plate screen easily. In thisway, there is no, or minimal, fly ash buildup on the catalyst platescreen. In contrast, the catalyst wire mesh screen is a rough finishmaterial that is placed above the catalyst bed, resulting in fly ashaccumulating in and on this rough finish material. Fly ash can get intothe mesh, get trapped and thereby create piles of fly ash.

The smooth finished surface of the catalyst plate screen according toembodiments of the invention provides a further benefit in that itincreases the cleaning range of air cannons because the pressuredifferential of the catalyst plate screen keeps the air cannon blast onthe screen for a further distance as compared with the catalyst wiremesh screen. Similarly, sonic horns are more effective when cleaning thecatalyst plate screen as compared to the catalyst wire mesh screenbecause of the catalyst plate screen's smooth surface. The sonic hornshave limited effectiveness in cleaning the catalyst wire mesh screenbecause the catalyst wire mesh screen has a rough surface and, as aresult, the sonic horns don't move the fly ash enough to get it off thecatalyst wire mesh screen. Instead, often, the sonic horns pack the flyash into the rough areas of the catalyst wire mesh screen. In thissense, the catalyst plate screen provides a further benefit in that itdoes not allow the packing of fly ash as compared with the catalyst wiremesh screen.

A further benefit of the catalyst plate screen according to embodimentsof the invention is that it aids the flow of flue gas. In contrast, thecatalyst wire mesh screen does not aid and has minimal to no effect onflue gas flow. Thus, plugging and creation of ash piles tend to developfor catalyst wire mesh screens, which in turn results in increase influe gas velocity and angled flow which causes the catalyst to erode.The catalyst plate screen, according to embodiments of the invention, asa result of its flow aid properties, normalizes air flow as discussedabove. Further, the catalyst plate screen's resistance to fly ashbuildup avoids such flue gas velocity increase and angled flow. With thecatalyst plate screen according to embodiments of the invention, therecan be better control over the velocity angles, allowing flow to thecatalyst and protecting the catalyst from air cannon flow. Whereas, withthe catalyst wire mesh screen, there is little control over velocityangles resulting in flow from a variety of angles, which results inerosion. Thus, there is less catalyst erosion when the catalyst platescreen is used as compared with the catalyst wire mesh screen.

A further benefit of the catalyst plate screen according to embodimentsof the invention is that it protects the catalyst from air cannon blastsbetter than the catalyst wire mesh screen. The catalyst plate screenprovides this benefit over the catalyst mesh screen because of thehigher flow impacting surface area/area of holes and greater pressuredrop as compared with the catalyst wire mesh screen. Because of thehigher flow impacting surface area of the catalyst plate screen (moreopen area in the catalyst wire mesh screen), there is less opportunityfor the blast of air from the air cannon to reach and damage thecatalyst with the catalyst plate screen.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

What is claimed is:
 1. A system for use in a selective catalyticreduction reactor, the system comprising: a catalyst bed; and a screenlocated at a distance in a range of 1 in. to 12 ft. above the catalystbed so that flow of flue gas to the catalyst bed contacts the screen orpasses through the screen before the flow of flue gas contacts thecatalyst bed or passes through the catalyst bed, the screen adapted tosupport a weight of at least 400 pounds above the catalyst bed so thatthe weight is not imposed on the catalyst bed, the screen having aplurality of holes across its surface, wherein the screen is adapted tochange velocity distribution of the flue gas as it flows through thescreen, wherein the screen comprises a flat shape extending from one endof the screen to another end of the screen, the flat shape is defined ina plane perpendicular to the flow of flue gas, and wherein the change invelocity distribution as a result of the screen comprises a 3 to 6%decrease in Root Mean Square (RMS) of overall velocity distribution ofthe flue gas.
 2. The system of claim 1 further comprising: a ductadapted to channel the flue gas that emanates from a boiler to theselective catalytic reduction reactor.
 3. The system of claim 1, furthercomprising: a LPA separator for separating LPA from the flue gas priorto the flue gas contacting the screen.
 4. The system of claim 1, whereinthe screen comprises a perforated metal plate.
 5. The system of claim 1,wherein the plurality of holes comprises holes of different sizesarranged in a manner to achieve the change in the velocity distributionof the flue gas.
 6. The system of claim 1, wherein the plurality ofholes comprises holes of different sizes are arranged so that holesnearer to the perimeter of the screen are smaller than holes furtherfrom the perimeter of the screen.
 7. The system of claim 1, wherein thescreen comprises a surface having a roughness of 20 μm or less.
 8. Thesystem of claim 1, wherein the screen is adapted to normalize flue gasflow that has different velocities so that the screen increases thevelocity of a section of the flue gas flowing to the screen at avelocity lower than the flue gas's average velocity and decreases thevelocity of a section of the flue gas flowing to the screen with avelocity higher than the flue gas's average velocity.
 9. The system ofclaim 1 wherein the ratio of area of holes/area of screen surface is ina range of 45% to 65%.
 10. The system of claim 1, wherein the holes areshaped as a selection from the list consisting of: hexagon, circle,square, rectangular, triangle, pentagon, and combinations thereof. 11.The system of claim 1, wherein the screen comprises 12 gauge carbonsteel.
 12. A method for protecting a catalyst bed in a selectivecatalytic reduction reactor, the method comprising: disposing a screenat a distance in a range of 1 in. to 12 ft. above the catalyst bed sothat flow of flue gas to the catalyst bed contacts the screen or passesthrough the screen before the flow of flue gas contacts the catalyst bedor passes through the catalyst bed, the screen adapted to withstand aweight of at least 400 pounds without that weight being imposed on thecatalyst bed, the screen having a plurality of holes across its surface,wherein the screen comprises a flat shape extending from one end of thescreen to another end of the screen; flowing the flue gas through thescreen, wherein the flat shape is defined in a plane perpendicular tothe flow of flue gas; and changing velocity distribution of the flue gasas it flows through the screen, wherein the change in velocitydistribution as a result of the screen comprises 3 to 6% decrease inRoot Mean Square (RMS) of overall velocity distribution of the flue gas.13. The method of claim 12, wherein the screen comprises a perforatedmetal plate.
 14. The method of claim 12, wherein the screen comprises asurface having a R_(a) of 20 μm.
 15. The method of claim 12, wherein,when the flue gas flow has different velocities, normalizing the fluegas flow comprises increasing the velocity of a section of the flue gasflowing to the screen at a velocity lower than the flue gas's averagevelocity and decreasing the velocity of a section of the flue gasflowing to the screen with a velocity higher than the flue gas's averagevelocity.
 16. The method of claim 12, wherein the ratio of area ofholes/area of screen surface is in range of 45% to 65%.
 17. The methodof claim 12, wherein the holes are shaped as a selection from the listconsisting of: hexagon, circle, square, rectangular, triangle, pentagon,and combinations thereof.
 18. The method of claim 12, wherein the screencomprises 12 gauge carbon steel.